The present invention relates to a semiconductor device, and particularly, to a semiconductor device comprising a semiconductor pellet adhered to a portion for mounting the semiconductor pellet on a wiring substrate, an electrode of the semiconductor pellet being connected to an electrode of the wiring substrate, and the semiconductor pellet, the electrode thereof, and the electrode of the wiring substrate being sealed with a sealing material.
Semiconductor devices known heretofore include structures shown by FIG. 3. Referring to FIG. 3, a semiconductor device of this type comprises a wiring substrate 1 being a base made of a resin, a metal, a ceramic, etc., and having wiring formed with a copper foil and the like on the surface thereof, a semiconductor pellet 3 being adhered to a semiconductor pellet mounting portion 2 provided on the surface of the wiring substrate 1 by using an adhesive 4 interposed therebetween, an electrode 5 of the semiconductor pellet 3 and an electrode 7 provided on the wiring substrate 1 being electrically connected (by wire bonding) with each other using a gold wire 8, and a sealing material 11 comprising a thermosetting resin (or a resin such as a thermoplastic resin or a ultraviolet-curable resin) sealing the semiconductor pellet 3 and the wire-bonded portion (the electrode 5 of the semiconductor pellet 3, the electrode 7 provided on the wiring substrate 1, and the gold wire 8). The electrode 7 provided on the interconnecting substrate 1 and the semiconductor mounting portion 2 for mounting a pellet 3 are included in the wiring formed from a copper foil.
The wiring provided on the wiring substrate 1 above (such as the semiconductor pellet mounting portion 2, the electrodes 7, etc.) is formed generally by an etching process of the copper foil of a copper-clad laminated sheet. The copper-clad laminated sheet can be obtained by cladding a laminated sheet with a copper foil whose surface is roughened by a special treatment. Thus, the copper foil bites into the laminated sheet at the interface between the copper foil and the laminated sheet. The wiring substrate 1 exposes an irregular surface where no wiring is provided, i.e., the exposed surface 6 from which copper foil is removed.
In general, the semiconductor device above is electrically connected with another semiconductor device or a wiring substrate by means of a soldering process such as reflow soldering. Thus, to prevent the solder from adhering to the unnecessary portions of the surface of the wiring substrate 1, the portions are often coated with a protective film (a solder resist film) 10. Because the protective film 10 prevents a solder or a plating from adhering to the unwanted portions, in a conventional process, it is applied uniformly to the entire surface of the wiring substrate 1 except for the semiconductor pellet mounting portion 2 thereon and the electrode 7.
In a conventional semiconductor device as described above with reference to FIG. 3, the semiconductor pellet 3 and the connected portion (comprising the electrode 5 of the semiconductor pellet 3, the electrode 7 on the wiring substrate 1, and the gold wire 8) by wire bonding are covered with a sealing material 11 to be sealed by hardening the sealing material 11 by means of, for instance, heating (or cooling, or an ultraviolet irradiation). However, the cooling after heating caused warping of the wiring substrate 1 due to the difference in coefficient of thermal expansion between the interconnecting substrate 1 and the sealing material 11. FIG. 4A is a cross sectional view of a wiring substrate 1 on which the problematic warping occurred. Such a warping causes separation of the sealing material 11 from the surface of the wiring substrate 1 to produce cracks at the interface 12 between the wiring substrate 1 and the sealing material 11. Accordingly, a means of preventing warping from occurring has been demanded.
In a conventional semiconductor device, furthermore, the sealing material 11 and the interconnecting substrate 1 absorb moisture from the air, and the thus absorbed moisture is vaporized to expand when heated during reflow soldering. This, as shown in FIG. 4B, causes separation of the inherently inferior adhesive interface 12a between the adhesive 4 and the semiconductor pellet mounting portion 2. This separation cannot be ignored because it may lead to further interface separation 13 between the sealing material 11 and the protective film 10 or to cohesive failure 14 of the protective film 10. Such interface separation 13 and cohesive failure 14 are referred to collectively as reflow cracks.
Some technical proposals are made to overcome the problems above in unexamined published Japanese utility model applications Nos. Hei 5-48338 through Hei 5-48344.
The proposed technique comprises connecting the electrode of a semiconductor pellet mounted on a wiring substrate with the electrode of the wiring substrate, providing a frame-like or rod-like protrusion, groove, or a complex structure thereof on the outer side of the connecting portion so as to surround the semiconductor pellet, and sealing the semiconductor pellet and the like inclusive of such protrusion, groove, etc. Providing such a groove extends a path for moisture entering the semiconductor device, and hence, it takes a longer duration of time before reaching the semiconductor pellet. Furthermore, the protrusion increases the adhesiveness between the sealing material and the wiring substrate to prevent separation from occurring therebetween. Thus, as a whole, a semiconductor device with a longer life can be provided.
However, in the fabrication of the semiconductor device above, an additional step of providing a frame-like or rod-like groove or protrusion is required. Such an additional step of fabrication is greatly disadvantageous because it results in a process with increased steps and cost of production. Furthermore, it has no effect on preventing reflow cracks from generating.
The present invention has been accomplished with an aim to overcome the aforementioned problems. Accordingly, the present invention provides a semiconductor device comprising a semiconductor pellet adhered to a semiconductor pellet mounting portion provided on a wiring substrate, an electrode of the semiconductor pellet and an electrode on the wiring substrate being connected to each other, and a sealing material, sealing the semiconductor pellet, the electrode thereof, and the electrode on the wiring substrate, in which device reflow cracks such as interface separation between the sealing material and the wiring substrate or the cohesive failure of the sealing material, which are due to the expansion of the vaporized moisture absorbed by the sealing material or the wiring substrate in hot reflow soldering, are prevented from being generated and an improved reliability and a longer life are provided.